Invading organisms such as bacteria produce toxins that damage host tissues and interfere with normal metabolism; some toxins are actually enzymes that, by breaking down host tissues, prevent the localization of infections. Other bacterial substances destroy the host's phagocytes. Viruses and retroviruses are parasitic on host cells, causing cellular degeneration, as in rabies, poliomyelitis, and AIDS, or cellular proliferation, as in warts and cold sores. Some viruses have been associated with the development of certain cancers. Substances produced by many invading organisms cause allergic sensitivity in the host; the immune response to virus infection has been implicated in some diseases (see allergy).

Infections may be spread via respiratory droplets, direct contact, contaminated food, or vectors, such as insects. They can also be transmitted sexually (see sexually transmitted diseases) and from mother to fetus. Immunity is the term used to describe the capacity of the host to respond to infection. Drugs that help fight infections include antibiotics and antiviral drugs.